Metal oxide 247 superconducting materials

ABSTRACT

The present invention comprises novel oxide materials exhibiting bulk superconductivity up to and exceeding 85K and processes for their synthesis. The oxides are within the formula RaBabCucOd wherein 1.9&lt;a&lt;2.1, 3.9&lt;b&lt;4.1, 6.8&lt;c&lt;7.2, 14.4&lt;d&lt;15.2 and wherein R is Y or any of the lanthanide rare earth elements. Certain substitutions such as Ca and La on the R and Ba sites are included.

This application is a Reissue of Ser. No. 07/560,033 (filed Jul. 30,1991, now U.S. Pat. No. 5,140,000). .Iaddend.

The present invention comprises novel oxide materials exhibiting bulksuperconductivity up to 85K and certain of which exhibitsuperconductivity at temperatures exceeding 85K, and processes for theirsynthesis.

The oxide compounds RBa₂ Cu₃ O₇₋δ (hereinafter referred to as 1-2-3) andRBa₂ Cu₄ O₈₋δ (hereinafter referred to as 1-2-4) are known to besuperconductors with superconducting transition temperatures T_(c),respectively, of 90-93K (when δ<0.15) and of about 79-81K. For 1-2-3, δmay range from 0 to 1.0 while for 1-2-4, δ cannot be varied much at alland lies close to zero. The structure of 1-2-3 is equivalent to atripled defect perovskite cell with consecutive layers in the unit cellof: R in a B-site with vacant oxygen sites in the layer, a buckledsquare planar CuO₂ layer with Cu in the corner-shared A-sites, a BaOlayer with Ba in the B-site, a square-planar CuO₁₋δ layer with Cu in thecorner-shared A-sites, then the structure repeated in reverse order by areflection about the CuO₁₋δ layer. This layer can load or unload oxygen,as described by the value of δ, depending on the temperature ofannealing conditions and the ambient oxygen partial pressure. Thesematerials are typically prepared by solid state reaction at hightemperature of precursor materials, such as Y₂ O₃, BaCO₃, and CuOfollowed by annealing at lower temperatures, about 400° C. in an ambientoxygen containing atmosphere. When fully loaded (δ=0), T_(c) is amaximum around 92K when R is Y or any of the lanthanide rare-earthelements. In this state the oxygens in the CuO₁₋δ layer are ordered ontoone set of sublattice sites forming -Cu-O-Cu-O- chains, the otherwisecrystallographically equivalent sites being vacant. This orderingrenders the material orthorhombic in symmetry. If the anneal temperatureis raised above 400° C. these oxygens commence to unload from thematerial (δ>0) and begin to disorder onto the otherwise vacant sitesuntil, at a critical temperature T_(OT), where both sites finally haveequal random occupancy, the material undergoes a second-order transitionfrom orthorhombic to tetragonal symmetry.

This transition presents problems in synthesizing and processing thematerials for optimum performance. Typically 1-2-3 is oxygen loadedafter synthesis to maximise the superconducting transition temperatureby slow cooling or annealing in an oxygen containing atmosphere totemperatures below 450° C. As the material cools through the transitiontemperature T_(OT) the thermal expansion becomes large and highlyanisotropic resulting in extensive microcracking. This reduces themaximum current-carrying capacity (the so-called critical current) aswell as reducing the mechanical strength. The oxygen diffusioncoefficient is also so low that oxygen loading occurs prohibitivelyslowly in dense material (<2% porosity).

The structure of 1-2-4 is the same as fully oxygen-loaded 1-2-3 (δ=0)with a double layer of CuO chains displaced 0.5b in the b-directionrelative to each other. Because of the stability of the doublechain-layer the oxygen stoichiometry remains nearly constant with δnearly zero independent of oxygen partial pressure and temperature. Thismaterial therefore does not present the same problem of the need to loadoxygen in order to prepare superconducting material and its associatedproblems of a high thermal expansion coefficient and accompanyingmicro-cracking. Unfortunately the transition temperature is too low tobe of practical use at the temperature of liquid nitrogen (77K) due tothe deficient electron hole carrier concentration on the CuO₂ planes.

Intergrowths of 1-2-4 can occur in 1-2-3, and vice versa, and an orderedphase is known to exist having chemical formula R₂ Ba₄ Cu₇ O₁₅₋δ(referred to hereinafter as 2-4-7) which comprises alternating unitcells of 1-2-3 and 1-2-4 (Nature 334, 596 (1988). This reported compoundhad a low transition temperature T_(c) of 45 to 50K, and subsequentefforts by the authors failed to improve much on this.

It is an object of the present invention to provide novel materialsexhibiting superconductivity. Certain 2-4-7 materials of the inventionhave a superconducting transition temperature of 92K and certain otherdesirable properties including reduced oxygen loading requirements, areduced thermal expansion coefficient and a reduced tendency tomicrocrack during synthesis and processing.

It is a further object of the invention to enable preparation of 2-4-7materials substantially free from extended 1-2-3 or 1-2-4 intergrowthdefects.

The invention also provides processes for the preparation of the 2-4-7materials including processes which enable their preparation in oxygenat ambient atmospheric pressure.

These, and other aspects, features and advantages of the invention, willbecome more apparent in the detailed description with reference todrawings and examples which follows.

In broad terms the invention may be said to comprise oxide materialswhich exhibit bulk superconductivity within the formula

    R.sub.a Ba.sub.b Cu.sub.c O.sub.d

wherein:

    1.9<a<2.1,

    3.9<b<4.1,

    6.8<c≦7.2,

    14.4<d<15.2

R is L where L is Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm or Yb, or anycombination thereof,

Ba is Ba or Ba partially substituted by either or both of Sr and La,

Cu is Cu or Cu partially substituted by Li Ag, Au, Hg, Tl, Bi, Pb, Sb orGa or any Periodic Table transition metal element or Group 3a, 4a, or 5ametal, or any combination thereof, and

O is O or O partially substituted by any of N, P, S, Se or F or anycombination thereof.

The invention also encompasses such oxide materials wherein one or bothof L and Ba are partially substituted by any of the elements A given byCa, Li, Na, K, Cs or Rb, or any combination thereof. Such materialswherein A is Ca may be prepared to exhibit superconductivity at a Tc of85K or higher and exhibit enhanced oxygen mobility allowing oxygenloading in a time shorter than for the equivalent unsubstituted 2-4-7material.

Preferred materials have the formula

    L.sub.2-p Ba.sub.4-q A'.sub.p+q Cu.sub.7 O.sub.15-δ

wherein:

    0≦p+q<1,

    -0.2>δ>0.6,

L, Ba, A, Cu and O are as defined above and A' is La alone or La incombination with any other element of A.

Preferred materials of the invention have the formula L₂ Ba₄ Cu₇ O₁₅₋δ,where preferably δ>0.1 and preferably about O and L is preferably Y, Nd,Sm, Eu, Gd, Dy, Ho, Er or Tm or any combination thereof and mostpreferably Y or Er.

Particularly preferred alkali substituted materials of the inventionhave the formula

    L.sub.2-ax Ba.sub.4-(1-a)x Ca.sub.x Cu.sub.7 O.sub.15-δ

wherein:

a is 0.75,

x is 0.025, 0.05, 0.1 or 0.2, and

L is either Y or Er.

Such materials include those having the formula L_(2-p) Ca_(p) Ba_(4-q)La_(q) Cu₇ O₁₅₋δ, including those wherein L is Y, Nd, Sm, Eu, Gd, Dy,Ho, Er or Tm.

Further preferred materials are those having the formula L₂ Ba_(4-q)La_(q) Cu₇ O₁₅₋δ, of which a particularly preferred example is Y₂ Ba₃.8La₀.2 Cu₇ O₁₅₋δ.

The invention also comprises a process for preparing the oxide materialscomprising reacting precursor materials for between 1 and 300 hours at atemperature T (in units of °C.) and oxygen partial pressure Po₂ (inunits of Pa) satisfying the equation .[.1210-180L+21L²<T<2320-581.5L+58.5L²⁻ .]. .Iadd.1210-180L+21L² <T<2320-581.5L+58.5L².Iaddend.where

    L=log.sub.10 Po.sub.2

and preferably wherein Po₂ ≦10⁶ Pa, and most preferably wherein Po₂ issubstantially 10⁵ Pa and 845° C.≦T≦870° C.

Materials of the general formula of the invention where R is Y, Ba isthe element Ba, Cu is the element Cu and O is the element 0, and whichhave been oxygen loaded to the point where δ is reduced to approximatelyzero, exhibit superconductivity excceding 90K.

Of the .[.1-2-3-.]. .Iadd.1-2-3 .Iaddend.and 1-2-4 units .Iadd.in.Iaddend.2-4-7 only the 1-2-3 units will load or unload oxygen. As aconsequence the oxygen variability for 2-4-7 is halved relative to 1-2-3and potential for cracking is therefore diminished. Moreover, 2-4-7remains orthorhombic over its entire oxygen composition range, thusavoiding the above mentioned problems of the orthorhombic-to-tetragonaltransition which occurs in 1-2-3. The thermal expansion coefficient istherefore not strongly anomalous nor highly anisotropic and thepropensity for cracking is greatly diminished. In general, orderingdefects will occur resulting in additional intergrowths of either 1-2-3or 1-2-4, or both in 2-4-7 so that the Cu stoichiometry in the chemicalformula will not usually be exactly 7.

In the accompanying drawings that are referred to in the examples:

FIG. 1 shows the phase diagram for the Y-Ba-Cu-O system showing thestability regions for 1-2-3 and 2-4-7 (shaded region) as a function ofoxygen partial pressure Po₂ and temperature. The dashed line shows theO-T transition in metastable 1-2-3 and the oblique sloping lines showcontours of constant composition in metastable 1-2-3 with values of δshown.

FIG. 2 shows an X-ray diffraction pattern using Co K.sub.α radiation forY₂ Ba₄ Cu₇ O₁₅₋δ. Impurity lines are marked X for BaCuO₂ and O for Y₂BaCuO₅.

FIG. 3 shows the temperature dependence of the AC susceptibility for Y₂Ba₄ Cu₇ O₁₅₋δ annealed in oxygen at various temperatures shown, thenquenched into liquid nitrogen.

FIG. 4 shows the onset temperature T_(c) for the diamagnetic onset,obtained from measurements such as shown in FIG. 3, as a function of δ.

FIG. 5 shows the orthorhombic distortion in quenched samples of Y₂ Ba₄Cu₇ O₁₅₋δ as a function of δ.

FIG. 6 shows the change in molar volume per formula unit for quenchedsamples of Y₂ Ba₄ Cu₇ O₁₅₋δ as a function of δ.

FIG. 7 shows the change in molar volume as a function of δ for 1-2-3 perY₂ Ba₄ Cu₆ O₁₄₋δ formula unit. Open symbols: dilation on quenching;solid symbols; X-ray and neutron powder diffraction data from PhysicalReview B39, 2784(1989).

FIG. 8 .[.shows.]. .Iadd.show .Iaddend.CoK.sub.α X-ray diffraction (XRD)patterns for yttrium 1-2-3(a), 2-4-7(b), and 1-2-4(c).

The materials of the invention may be prepared as a thin film usingknown techniques, or as a bulk materials (including thick films). Thematerials of the invention may be prepared by solid state reaction andsintering of the appropriate precursor materials by techniques known inthe art for preparation of 1-2-3, but with additional reference to thechoice of oxygen partial pressures and temperatures for carrying out thereaction as are further described. Preparation of 1-2-3 is described inD W Murphy et al, Science 241, 922 (1988) for example. Alternatively theprecursor materials may be stoichiometrically mixed as nitrates inaqueous or other suitable solution and sprayed as a mist which is passedthrough an oven, furnace, microwave heating zone or the like for rapidreaction of the discrete droplets. The reacted droplets or particles maythen be collected by way of a cyclone, filter, electrostaticprecipitator, or the like. The fine reacted particles thus produced maybe sintered into a body of arbitrary shape by heating at temperaturesand oxygen partial pressures within the 2-4-7 stability region furtherdescribed below.

The substituted 2-4-7 materials of the invention may be prepared whenthe reaction and sintering are carried out at a temperature T and oxygenpartial pressure Po₂ which satisfy the equation

    1220-180L+21L.sup.2 <T<2320-581.5L+58.5L.sup.2

where L=log₁₀ Po₂.

Referring to FIG. 1, this formula defines the shaded region marked `247`which defines the 2-4-7 stability boundary with respect to other phases.By reacting and sintering within this band, material with compositionclose to 2-4-7 may be prepared and, by extended annealing, defectintergrowths of 1-2-3 and 1-2-4 may be minimised. As is known in theart, at intermediate steps the material should preferably be ground andmilled and optionally recompressed to increase the homogeneity beforesubjecting to further reaction and sintering within the stability band.For example, Y₂ Ba₄ Cu₇ O₁₅₋δ may be prepared in 1 bar of flowing oxygenbetween temperature of 845° C. and 870° C. though, in general, the exactlocation of these boundaries will depend upon the composition and degreeof elemental substitution. Another novel preparation technique is toreact, sinter or otherwise thermally process at the solidus meltboundary of the 2-4-7 stability band in order to achieve grain growth,grain orientation and densification, a process described as melttexturing. A further alternative is that prior to the last sinteringstep, the grains of the powdered 2-4-7 may be crystallographicallyaligned in a strong magnetic field according to the known art and thensintered to produce a preferentially oriented ceramic.

Material prepared at the lower temperature/lower pressure end of thestability band may be porous and not ideally sintered. Porosity may bereduced by using solgel, coprecipitation, spray drying of aqueousprecursor solution, spray pyrolysis or other methods as are known in theart of ceramics synthesis. The material may also be densified by raisingthe temperature outside of the stability band for short duration. Theinitial sintering rate is faster than the decomposition rate anddensification occurs. The material should typically, for example, befurther annealed within the stability band subsequent to densificationand several densification cycles could, for example, be employed. Foradvanced densification either the temperature or duration of sinter willneed to be so large that 2-4-7 will begin to decompose to 1-2-3+copperoxide but, on further extended annealing within the stability band,2-4-7 will regrow especially if the precipitates of copper oxides arecontrolled to be finely dispersed. Oxygen partial pressure may becontrolled by gas pressure, or alternatively, across the entirestability band shown in FIG. 1, by the use of electrochemical means tocontrol the oxygen activity in the 2-4-7, for example, by placing anoxygen-ion electrolyte conductor such as Y-stabilised ZrO₂ in contactwith the material and maintaining an appropriate voltage across the cellthus formed according to the known methods of solid-state electrolyticcells.

The reaction rate may be enhanced by the use of certain alkali metalfluxes, catalysts or reaction rate enhancers which may operate byproviding a molten or vapour phase flux or by temporary or permanentsubstitution into the lattice of the reactants or of the final product.Such fluxes or catalysts etc for the preparation of 2-4-7 include: theoxides, carbonates, halides and hydroxides of the alkali metals.Preferred examples of catalysts are the oxides of Na and K which may beintroduced to the precursor materials as NaNO₃ or KNO₃ which willdecompose to the oxides. The attractive feature of these catalysts isthat they are volatile and will, with time, evaporate off leavingphase-pure 2-4-7 material. Further catalyst may be added as required atintermediate grinding and milling steps. The alkali catalysts appear tooperate, at least in part, by temporary or permanent substitution intothe 2-4-7 lattice, predominantly in the Ba-site and also in the R-site.The use of alkali carbonates as catalysts has been described in relationto the synthesis of YBa₂ Cu₄ O₈ (Nature 338, 328 1989)). The catalystremained in the solid state during reaction and required to be removedby dissolving out in water at the completion of synthesis leavingpowdered 1-2-4 material only. In the method of the present inventionsmall amounts only of alkali catalyst are employed, preferably theoxides of Na or K which evaporate away during synthesis leaving sinteredceramic product. The mole fraction, α of introduced catalyst ispreferably in the range O<α≦1.0 and most preferably 0.1≦α≦0.3. Thesynthesis of ceramic product, as opposed to powder, is a major advantageof this technique. Another method for enhancing the reaction rate is tosubstitute Ca into the lattice using known methods of chemicalpreparation techniques. As for 1-2-3, Ca will substitute into 2-4-7predominantly in the R-site but also in the Ba-site, the substitutedmaterial exhibiting enhanced atomic diffusion rates.

Preferred examples of 2-4-7 oxide materials include L₂ Ba₄ Cu₇ O₁₅₋δ, L₂Ba₄ Cu₇ O₁₅, L_(2-p) Ba_(4-q) Ca_(p+q) Cu₇ O₁₅₋δ where 0≦p+q≦0.6,L_(2-p) Ba_(4-q) Na_(p+q) Cu₇ O₁₅₋δ, L_(2-p) Ba_(4-q) K_(p+q) Cu₇ O₁₅₋δ,L₂ Ba_(4-q) La_(q) Cu₇ O₁₅₋δ and L_(2-p) Ca_(p) Ba_(4-q) La_(q) Cu₇O₁₅₋δ. Moreover as T_(c) in Y₂ Ba₄ Cu₇ O₁₅₋δ increases monotonicallytowards 92K if δ is decreased towards 0 it is clear that T_(c) may beincreased above 92K if δ is reduced below zero by excess oxygen loadingor if the hole concentration is otherwise increased. Examples of suchnovel materials are

L₂ Ba₄ Cu₇ O₁₅₋δ with -0.2<δ<0.0 prepared, for example, by slow coolingat oxygen pressures in excess of 10⁵ Pa or by the use of electrochemicaltechniques as described above. A preferred example is when L=Nd. Thislarge ion increases the a and b lattice parameters thus reducingoxygen-oxygen repulsion and allowing the insertion of extra oxygen;

    L.sub.2-p A.sub.p Ba.sub.4-q A.sub.q Cu.sub.7 O.sub.15-δ  as described above;

L₂ Ba₄ Cu_(7-w) T_(w) O₁₅₋δ where T is any of, or combination of Li, Ag,Au, Hg, or Tl in their monovalent states and preferably substituted onthe copper chain-sites.

The materials of the invention and their preparation are furtherillustrated by the following examples.

EXAMPLE 1

Samples of Y₂ Ba₄ Cu₇ O₁₅₋δ were prepared by reaction betweentemperatures of 840° C. and 870° C. of stoichiometric quantities of Y₂O₃, Ba(NO₃)₂ and submicron sized CuO in flowing oxygen at 1 bar. Thephase diagram shown in FIG. 1 shows the region of stability of 2-4-7thus determined. This together with data reported at high pressuresbetween 20 and 100 bar (Physica C159, (1989) 287) allows theconstruction of the boundaries of stability of 2-4-7. Samples preparedwithin the boundaries indicated will progress to the requiredsingle-phase products given sufficient reaction time. The precursormaterials were mixed with 0.2 gram formula units of KNO₃ or NaNO₃ anddecomposed at 750° C. for 1 hour. The residue was pressed into pelletsand reacted as above. At 12, 24 and 36 hours the pellets were rapidlywithdrawn from the furnace, ground and milled and re-pressed intopellets for further reaction, then left for a further 3 to 5 dayssintering under the same reaction conditions. The result was nearlysingle-phase material as indicated by the X-ray diffraction patternsshown in FIG. 2. In order to control the oxygen content samples wereannealed at a fixed oxygen partial pressure and a given temperature thenrapidly quenched by dropping into liquid nitrogen. FIG. 3 shows ACmagnetic susceptibility measurements on a sample annealed in oxygen atthe different temperatures shown. Weight changes in these samples weremeasured and it was found that a sample fully oxygen loaded at 350° C.then subjected to an anneal in argon at 550° C. experienced a masschange corresponding to a change in δ of 0.99. This gave an absolutescale to determine δ. FIG. 4 shows the onset temperature, T_(c) fordiamagnetic susceptibility plotted against δ, illustrating T_(c) risingmonotonically with δ, in contrast to the known behaviour for 1-2-3 whichexhibits plateaux. X-ray diffraction measurements were performed onquenched samples and FIG. 5 shows the orthorhombic distortion(b-a)/(b+a) as a function of δ. Orthorhombicity is never lost even whenfully loaded. FIG. 6 shows the change in molar volume for Y₂ Ba₄ Cu₇O₁₅₋δ as a function of δ determined from the reversible changes inlength of the quenched samples. The change in volume per formula unit is3.1A³ per oxygen vacancy. The increase in volume with δ exactly matchesthat for 1-2-3 shown in FIG. 7. For 1-2-3 the volume change per formulaunit Y₂ Ba₄ Cu₆ O₁₄₋δ is 3.6A³ per oxygen vacancy. As only half as muchoxygen loads into 2-4-7 as 1-2-3, the additional anomalous thermalexpansion due to oxygen loading will be half that of 1-2-3. The absenceof a tetragonal to orthorhombic transition means that the thermalexpansion coefficient is free of the highly anisotropic behaviourobserved in 1-2-3 just below the transition which is a major drivingforce for microcracking in 1-2-3. Microcracking is therefore greatlyreduced in 2-4-7.

Single phase material lacking in any impurity phases evident from X-raydiffraction was prepared by these methods. .[.FIG. 8 shows.]..Iadd.FIGS. 8a to 8c show .Iaddend.XRD patterns Y₂ Ba₄ Cu₇ O₁₅ comparedwith that for the 1-2-3 and 1-2-4 compounds.

EXAMPLES 2 TO 10

These samples were prepared as described in Example 1 using 0.2 molefraction of NaNO₃ as the reaction rate enhancer and reacted in flowingoxygen at one atmosphere at temperatures between 860° and 870° C.Calcium and lanthanum were introduced as their nitrates and reactiontime was between 3 and 5 days with 3 or more intermediategrinding/milling steps. The results are summarised under Table 1. Thesuperconducting transition temperature T_(c) is reported as the highesttemperature for zero electrical resistivity, which usually coincidedwith the onset of diamagnetic susceptibility, and the c-axis latticeparameter is also tabulated. All samples were slow-cooled in oxygen to350° C. and held there overnight. The degree of oxygen loading arisingfrom this annealing does not necessarily correspond to that required formaximum T_(c) values.

                  TABLE I                                                         ______________________________________                                        Ex.  Composition (atomic ratio)                                                                           Tc (K)                                            No.  Y      Er     Ca      Ba   La  Cu  (R = 0)                                                                              c (Å)                      ______________________________________                                        2    2                     4        7   93     50.603                         3           2              4        7   92                                    4           1.925  .1      3.975    7   91                                    5           1.85   2       3.95     7   91                                    6    1.981         .[..25.]. .025                                                                        3.994    7   88.3                                  7    1.962         .05     3.988    7   91     50.613                         8    1.925         .1      3.975    7   91     50.608                         9    1.85          .2      3.95     7   88     50.621                         10   2                     3.8  0.2 7   76     50.524                         ______________________________________                                    

The foregoing describes the invention including preferred forms andexamples thereof. The preparation of derivative materials for formsother than sintered ceramic form, i.e. thin films, thick films, singlecrystals, filaments and powders other than those specificallyexemplified will be within the scope of those skilled in the art in viewof the foregoing. The scope of the invention is defined in the followingclaims.

We claim:
 1. Oxide materials which exhibit bulk superconductivity at.[.temperatures exceeding 85K.]. .Iadd., temperature of at least88K.Iaddend., within the formula

    R.sub.a B.sub.b Cu.sub.c O.sub.15δ

wherein:

    1.9<a<2.1,

    3.9<b<4.1,

    6.8<c≦7.2,

    .[.-0.2<δ<0.6.]. .Iadd.-0.2<δ<0.1.Iaddend.,

R is Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm or Yb, or any combinationthereof, and B is Ba or Ba plus a minor amount of Sr or La or acombination thereof.
 2. The material of claim 1, wherein a=2, b=4, c=7and having the formula

    .[.R.sub.2 R.sub.4 Cu.sub.7 O.sub.15-δ

and wherein -0.2<δ<0.6.]. .Iadd.R₂ B₄ Cu₇ O₁₅₋δ .Iaddend..
 3. Thematerial of claim 2, wherein .[.-0.2<δ<0.1.]. .Iadd.-0.2<δ<0.0.Iaddend..4. The material of claim 2, wherein δ is about 0 and R is Y, Nd, Sm, Eu,Gd, Dy, Ho, Er or Tm or any combination thereof.
 5. The material ofclaim 2 wherein R consists essentially of Y or Er.
 6. Oxide materialswhich exhibit bulk superconductivity at temperatures exceeding 85K,within the formula

    R.sub.a-p B.sub.b-q A.sub.p+q Cu.sub.c O.sub.15-δ

wherein:

    1.9<a<2.1,

    3.9<b<4.1,

    6.8<c≦7.2,

    .[.O<p+q<1.]. .Iadd.0<p+q<1.Iaddend., .Iadd.p>0 .Iaddend.

    -0.2<δ0.6,

R is Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm or Yb, or any combinationthereof, B is Ba or Ba plus a minor amount of Sr or La or a combinationthereof, and A is Ca, Li, Na, K, Cs or Rb or any combination thereof, orLa or La in combination with any of Ca, Li Na, K, Cs and Rb.
 7. Thematerial of claim 6, wherein A is Ca.
 8. Oxide materials which exhibitbulk superconductivity at temperatures exceeding 85K within the formula

    R.sub.2-ax B.sub.4-(1-a)x Ca.sub.x Cu.sub.7 O.sub.15-δ

wherein: a is 0.75, x is 0.025, 0.05, 0.1 or 0.2, -0.2<δ<0.6, R consistsessentially of Y or Er, and B is Ba or Ba plus a minor amount of Sr orLa or a combination thereof.
 9. Oxide materials which exhibit bulksuperconductivity at temperatures exceeding 85K within the formula

    R.sub.2-p Ca.sub.p B.sub.4-q La.sub.q Cu.sub.7 O.sub.15-δ

wherein:

    -0.2<δ<0.6,

    .[.0≦p+q<1.]. .Iadd.0<p+q<1, g>0.Iaddend.,

R is Y, La, Nd, Sm, Eu, Gd, Dy, Er, Tm, or Yb, or any combinationthereof, and B is Ba or Ba plus a minor amount of Sr.
 10. The materialsof claim 9 wherein R is Y, Nd, Sm, Eu, Gd, Dy, Ho, Er or Tm.
 11. Thematerials of claim 9 having the formula R₂ Ba_(4-q) La_(q) Cu₇ O₁₅₋δ,wherein -0.2<δ<0.6 and O≦p+q<1.
 12. The material of claim 11, having theformula Y₂ Ba₃.8 La₀.2 Cu₇ O₁₅₋δ wherein -0.2<δ<0.6.
 13. A process forpreparing oxide materials which exhibit bulk superconductivity at.[.temperatures exceeding 85K.]. .Iadd.a temperature of at least 88K.Iaddend.within the formula

    R.sub.a B.sub.b Cu.sub.c O.sub.15-δ

wherein:

    1.9<a<2.1,

    3.9<b<4.1,

    6.8<c≦7.2,

    .[.-0.2<δ<0.6.]. .Iadd.-0.2<δ<0.1.Iaddend.,

R is Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm or Yb, or any combinationthereof, and B is Ba or Ba plus a minor amount of Sr or La or acombination thereof, comprising reacting precursor materials for between1 and 300 hours at a temperature T (in units of °C.) and oxygen partialpressure Po₂ (in units of Pa) satisfying the equation

    1210-180L+21L.sup.2 <T<2320-581.5L+58.5L.sup.2 where L=log.sub.10 Po.sub.2.


14. A process for preparing oxide materials which exhibit bulksuperconductivity at temperatures exceeding 85K within the formula

    R.sub.a-p B.sub.b-q A.sub.p+q Cu.sub.c O.sub.15-δ

wherein:

    1.9<a<2.1,

    3.9<b<4.1,

    6.8<c≦7.2,

    .[.O≦p+q<1.]. .Iadd.0<p+q<1, p>0, and.Iaddend.,

    .[.-0.2<δ<0.6.]. .Iadd.-0.2<δ<0.1.Iaddend.,

R is Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm or Yb, or any combinationthereof, B is Ba or Ba plus a minor amount of Sr, La, or a combinationthereof, A is Ca, Li, Na, K, Cs or Rb or any combination thereof or Laor La in combination with any of Ca, Li, Na, K, Cs, and Rb comprisingreacting precursor materials for between 1 and 300 hours at atemperature T (in units of °C.) and oxygen partial pressure Po₂ (inunits of Pa) satisfying the equation

    121- 180L+21L.sup.2 <T<2320-581.5L+58.5L.sup.2 where L=log.sub.10 Po.sub.2.


15. The process according to claim 14 wherein .[.0<p+q<1 and.]. A is Ca.16. The process according to .[.either one of claims 13 and 14.]..Iadd.claims 13, 14, or 29.Iaddend., wherein Po₂ ≦10⁶ Pa.
 17. Theprocess according to .[.either of claims 13 and 14.]. .Iadd.claims 13,14, or 29.Iaddend., wherein Po₂ is substantially 10⁵ Pa and 845°C.≦T≦870° C.
 18. The process according to .[.either of claims 13 and14.]. .Iadd.claims 13, 14, or 29.Iaddend., wherein the precursormaterials are reacted together with an alkali flux, catalyst, orreaction rate enhancer comprising a nitrate, oxide, chloride, hydroxideor carbonate of any of Li, Na, K, Rb or Cs or any combination thereof.19. The process according to .[.either of claims 13 and 14.]..Iadd.claims 13, 14, or 29.Iaddend., wherein the precursor materials arereacted together with an alkali flux, catalyst, or reaction rateenhancer comprising a nitrate, oxide, chloride, hydroxide or carbonateof any of Li, Na, K, Rb or Cs or any combination thereof, and whereinthe flux, catalyst or reaction rate enhancer is introduced as a molefraction of the precursor materials between 0 and 1.0.
 20. The processaccording to .[.either of claims 13 and 14.]. .Iadd.claims 13, 14, or29.Iaddend., wherein the precursor materials are reacted together withan alkali flux, catalyst, or reaction rate enhancer comprising anitrate, oxide, chloride, hydroxide or carbonate of any of Li, Na, K, Rbor Cs or any combination thereof, and wherein the flux, catalyst orreaction rate enhancer is introduced as a mole fraction of the precursormaterials between 0.1 and 0.3.
 21. A process according to .[.either ofclaims 13 and 14.]. .Iadd.claims 13, 14, or 29.Iaddend., wherein theoxygen partial pressure Po₂ is maintained by means of an electrochemicalcell or whereby the effective Po₂ is maintained by controlling thechemical thermodynamic oxygen activity in the material by means of anelectrochemical cell.
 22. A process according to .[.either of claims 13and 14.]. .Iadd.claims 13, 14, or 29.Iaddend., wherein periodicallyduring the reaction the reactants are cooled and reground or milled andthen recompacted.
 23. A process according to .[.either of claims 13 and14.]. .Iadd.claims 13, 14, or 29.Iaddend., including after forming thecation composition of the material altering or optimising the oxygencontent of the material by oxygen diffusion in or out of the material.24. A process according to .[.either of claims 13 and 14.]. .Iadd.claim23.Iaddend., wherein the material is prepared to comprise residualsubstituted alkali or Ca in the material, or is prepared at a lowsynthesis temperature such that the material is of a small grain size,to an extent that said alteration or optimisation of the oxygen contentis accelerated. .Iadd.25. Oxide materials which exhibit bulksuperconductivity at temperatures exceeding 85K, within the formula

    R.sub.a B.sub.b Cu.sub.c O.sub.15-δ

wherein:

    1.9<a<2.1,

    3.9<b<4.1,

    6.8<c≦7.2,

    -0.2<δ<0.0,

R is Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm or Yb, or any combinationthereof, and B is Ba or Ba plus a minor amount of Sr or La or acombination thereof. .Iaddend..Iadd.26. The material of claim 25,wherein a=2, b=4, c=7 and having the formula

    R.sub.2 B.sub.4 Cu.sub.7 O.sub.15-δ. .Iaddend..Iadd.27. The material of claim 26, wherein δ is about 0 and R is Y, Nd, Sm, Eu, Gd, Dy, Ho, Er or Tm or any combination thereof. .Iaddend..Iadd.28. The material of claim 26 wherein R consists essentially of Y or Er. .Iaddend..Iadd.29. A process for preparing oxide materials which exhibit bulk superconductivity at temperatures exceeding 85K within the formula

    R.sub.a B.sub.b Cu.sub.c O.sub.15-δ

wherein:

    1.9<a<2.1,

    3.9<b<4.1,

    6.8<c≦7.2,

    -0.2<δ<0.6,

T is Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm or Yb, or any combinationthereof, and B is Ba or Ba plus a minor amount of Sr or La or acombination thereof, comprising reacting precursor materials for between1 and 300 hours at a temperature T (in units of °C.) and oxygen partialpressure P₀₂ (in units of Pa) satisfying the equation

    121- 180L+21L.sup.2 <T<2320-581.5L+58.5L.sup.2

where L=log₁₀ P₀₂ and P₀₂ ≠ about 1 atmosphere. .Iaddend.